This invention relates to an image forming apparatus such as an electrophotographic copying machine or a printer.
In an image forming apparatus such as an electrophotographic copying machine or a printer, there is provided a sheet feed unit for containing paper sheets, and when an image is formed, a paper sheet is fed from the sheet feed unit, to form an image on the sheet.
FIG. 1 is a schematic drawing of a conventional image forming apparatus. In the drawing, the image forming apparatus 1 is equipped with the platen portion 2, the optical unit 3, the image forming means 4, the fixing unit 5, the sheet feed units 9a to 9d, reverse feed unit 9e, the manual sheet feed unit 9, the output tray 15. On the platen portion 2, it is placed an original document, of which an image is read through the optical unit 3, and the light of the read image is applied to the image carrying member 10 of the image forming means 4 to form a latent image, which is developed by the image forming means 4, to form a toner image. On the other hand, a paper sheet is conveyed out by the separation roller 140 from a sheet feed unit selected out of the sheet feed units 9a to 9d toward the conveyance rollers 132. The separation roller 140 has such a structure as not to rotate as long as a frictional force of a certain strength is not applied to it, and for example, if two sheets lying on each other are conveyed to the separation roller 140, the rotating force of the drive roller, which is placed opposite to the separation roller so as to press it, is not transmitted sufficiently to the separation roller 140, because the frictional force between the two sheets lying on each other is low. For this reason, the sheet in contact with the drive roller is conveyed, but the sheet in contact with the separation roller 140 is not conveyed because the separation roller does not rotate. The conveyance roller 132 conveys the paper sheet, which has been separated as a single sheet, to the loop forming rollers 134. The loop forming rollers 134 convey the sheet until the leading edge of the sheet hits the registration rollers 56 to form a loop, in order to correct the skew of the sheet. The registration rollers 56 start to rotate at a timing to make the sheet synchronized with the toner image on the image carrying member 10, and the toner image is transferred to the sheet. The paper sheet, having the toner image transferred, is conveyed to the fixing unit 5, in which the image is fixed, and is ejected onto the output tray 15.
In a conventional structure as described in the above, because loop formation is done by the conveyance of a paper sheet by the loop forming rollers 134 which are common to a plurality of sheet feed units, the amount of skew correction for a sheet is the same for every sheet feed unit.
However, the amount of skew of a sheet that is produced actually is different for each of the sheet feed units. As the cause of it, for example, the following can be thought of: the difference in the conveyance path length up to the loop forming rollers 134, the difference in the curvature of the conveyance path which is different depending on the position of the sheet feed units etc. For example, for the conveyance path from the sheet feed unit 9a arranged at the uppermost position to the loop forming roller 134, because of a small length of the path, the amount of skew component depending on the length of the path among the skew components produced by the friction between the sheet and the conveyance path can be suppressed comparatively to a low value, but the amount of skew component depending on the curvature of the path is comparatively large, because of the large curvature of the conveyance path. On the other hand, for the conveyance path from the sheet feed unit 9d arranged at the lowermost position to the loop forming roller 134, because of a large length of the path, the amount of skew component depending on the length of the path is comparatively large, but the amount of skew component depending on the curvature of the path can be suppressed comparatively to a low value, because of the small curvature of the conveyance path. However, in the case of a conventional apparatus, because it has a structure such that a loop is formed by the same loop forming rollers 134 in a sheet that is fed from any one of the sheet feed units 9a to 9d, the condition of the loop forming rollers 134 could not be set to an optimum one for each of the sheet feed units. Therefore, it has been the cause of lowering the reliability of sheet conveyance and the positional precision of the image on a sheet.
Further, in order to suppress the difference in the amount of skew of the sheet for each of the sheet feed units even to a small degree, it has been necessary to make the same the layout of the conveyance paths from the respective sheet feed units to the loop forming rollers 134. Accordingly, the degree of freedom in the design such as the arrangement of the conveyance paths in an image forming apparatus or the arrangement of the sheet feed units has been remarkably limited, which has been a cause to prevent an image forming apparatus from being made compact.
Further, because the roller 131 of the manual sheet feed unit 9 for manually feeding a sheet has a combined function of a separation roller for separating a sheet and a conveyance roller for conveying a sheet, its conveyance force is increased as compared to the separation roller of other sheet feed units. For this reason, the roller 131 of the manual sheet feed unit 9 has a structure different from the separation rollers, and that is a cause of the increase of the cost such as the designing cost and the material cost of the image forming apparatus.
Further, the roller for separating the sheets from the sheet feed unit is covered with a synthetic rubber, which is an elastic member, and the separation of sheets is performed by the frictional force between the elastic member and the sheet. Regarding the roller for separating sheets, in order to prevent that its capability of separation is lowered because of the deterioration of the frictional force of the elastic member by the change with the passage of time, it is necessary to periodically replace it with a new one.
FIG. 2 is a schematic drawing of a roller for separating a single sheet from others. For the replacement of the roller with a new one, it has been necessary that the rotary shaft 85 is dismounted from the main body of the image forming apparatus, and after that, the roller 86 covered with an elastic member is drawn out from the end of the rotary shaft 85, which necessitated a considerable time and labor.
It is an object of this invention to provide an image forming apparatus capable of making a correction for the skew of a sheet which is optimum for each of a plurality of sheet feed units.
The above-mentioned object is accomplished by any one of the following structures (1) to (6).
(1): An image forming apparatus including a plurality of sheet feed units for feeding a sheet, a single registration means provided at the upstream side of the image formation site where an image is formed on said sheet with respect to the direction of sheet conveyance, for conveying said sheet in accordance with the timing of forming the image on said sheet, and a plurality of loop forming means provided in correspondence with said plurality of sheet feed units respectively at the upstream side of said registration means with respect to said direction of sheet conveyance, wherein each of said plurality of loop forming means is controlled in a manner such that a loop is formed between itself and said single registration means.
(2): The image forming apparatus as set forth in the structure (1), further including a reversing mechanism for reversing said sheet upside down, said reversing mechanism having a reversing path capable of changing over the conveyance direction of said sheet to the reverse direction, wherein said sheet with its conveyance direction changed over by said reversing path is controlled in a manner such that a loop is formed between itself and said registration means by a loop forming means which is disposed closest to said reversing path among said plurality of loop forming means.
(3): The image forming apparatus as set forth in the structure (2), wherein the aforesaid reversing mechanism is provided in the main body of said image forming apparatus.
(4): The image forming apparatus as set forth in the structure (1), wherein each of the aforesaid plurality of sheet feed units includes a container portion for containing said sheet and conveying-out means for conveying out said sheet from said container portion, and each of said conveying out means provided in correspondence with each of said sheet feed units is provided at a position different from another with respect to a width direction of the image forming apparatus.
(5): The image forming apparatus as set forth in the structure (4), further including a reversing mechanism for reversing the aforesaid sheet upside down, wherein said reversing mechanism is disposed vertically in the side portion to the side of the aforesaid conveying-out means being disposed.
(6): The image forming apparatus as set forth in the structure (1), wherein the aforesaid plurality of loop forming means are set in a condition of different sheet gripping forces respectively in accordance with the conveyance paths from the respective loop forming means to the aforesaid registration means.
Further, desirable structures (7) to (10) are as follows.
(7) An image forming apparatus comprising a plurality of sheet feed units for feeding a sheet, a pair of registration rollers for carrying out the conveyance of a sheet in synchronism with the rotation of an image carrying member, and a plurality of pairs of loop forming rollers provided at the upstream side of said pair of registration rollers with respect to the direction of sheet conveyance for conveying a sheet by rotation, characterized by it, that said plurality of pairs of loop forming rollers are provided in correspondence with said plurality of sheet feed units, and convey a sheet until the leading edge of the sheet hits said pair of registration rollers to form a loop in the sheet before said pair of registration rollers start to rotate.
(8) The image forming apparatus as set forth in the paragraph (7), further comprising a reversing path in which the first surface and the second surface, that is, the front side and the rear side, of a sheet characterized by it, that the formation of a loop of a sheet to be subjected to an image formation on the second surface after an image has been formed on its first surface is carried out by a pair of loop forming rollers corresponding to the sheet feed unit disposed closest to said reversing path.
(9) An image forming apparatus comprising a pair of rollers having a structure such that a drive roller and a driven roller is pressed into contact by an urging member, and a sheet is conveyed by the rotation of said drive roller, characterized by it, that a brake member for limiting the rotation of said driven roller and said driven roller are built integrally and are made capable of being mounted to and dismounted from the main body of the apparatus through an elastic member having resilient force.
(10) The image forming apparatus as set forth in the paragraph (9) characterized by it, that the aforesaid driven roller is a separation roller for separating a single sheet from others.